1. Field of the Invention
The present invention relates to a novel process for making aluminum foil-filled plastic pellets for shielding against electromagnetic interference (EMI), and more particularly relates to a process for making such pellets which are filled with radially arranged aluminum foil by using two aluminum foil layers and a molten plastic matrix as raw materials.
2. Description of the Prior Art
In recent years, the progress of technology has led to a marked increase in the amount of sophisticated electronic equipment. However, the high-density electromagnetic waves produced from electronic equipment have the potential to damage or adversely affect the performance of other equipment or components. Also, exposure to electromagnetic waves may be harmful to the human body. Therefore, an electrically conductive outer shell is needed to shield against electromagnetic interference (EMI) produced by electronic equipment.
Heretofore, various methods have been used to shield electronic equipment. Metallic boxes and cans fabricated from steel, copper, aluminum, etc., have been used to surround high EMI emitters for the purposes of shielding. However, because shields fabricated from metal are cumbersome, heavy and costly, the electronics industry has resorted to metallized plating on plastics. However, the results obtained with metallic coatings have not always been satisfactory. In addition to being relatively uneconomical, once such metallic coatings are scratched through, they lose part of their shielding effectiveness. Unless such conductive coatings are continuous and free of voids, electromagnetic waves will be free to pass through. Frequently, it is difficult to obtain a dependable, 100% effective coating that is also resistant to peeling.
Further efforts by the electronics industry to develop more dependable light-weight materials for EMI shielding have led to a third approach, namely the use of electrically conductive component-filled plastic composites. It has been anticipated that it will be possible to mold intricate shapes from such composite materials by conventional means, yielding a finished part that promises to be more economical and dependable than metal or metal-coated plastics.
The principal factor influencing the performance of conductive component-filled plastic composites is the aspect ratio of the conductive fillers. The aspect ratio is defined as the ratio of the maximum dimension to the minimum dimension of the filler. For example, the aspect ratio of a fiber is the ratio of the length to the diameter of the fiber. According to electromagnetic wave percolation theory, if the conductive filler in the plastic retains a higher aspect ratio, the filler easily forms a conductive network; thus, the critical concentration of the conductive filler required to achieve the electromagnetic shielding effect (that is, the threshold percolation concentration) is reduced.
There are three types of methods for preparing conductive component-filled plastic composites. The first type involves compounding the conductive fillers in the form of powders, short fibers or flakes with the plastic matrix. Then, the mixture is hot-pressed molded or injection molded into various kinds of plastic products for shielding EMI.
For example, U.S. Pat. No. 4,474,685 discloses a process for fabricating electromagnetic shielding products by first compounding and then moulding a molding composition including a thermosetting resin binder and an electrically conductive filler (including carbon black, graphite and conductive metal powders). However, during the compounding with the resin matrix, the conductive powders may easily cluster, and thus are not capable of dispersing in the resin matrix. Consequently, the electromagnetic shielding effectiveness of the molded products can not be effectively improved. Furthermore, since the powder filler has a lower aspect ratio, according to the electromagnetic wave percolation theory as mentioned above, the amount of powder filler added (i.e., threshold percolation concentration) must be relatively high to achieve electrical conductivity. Consequently, the mechanical properties, color and other physical and chemical properties of the molded products are adversely affected.
Alternatively, if the conductive filler is in a higher aspect ratio form such as fibers or flakes, although the filler can be loaded to a lower level, clustering is still difficult to prevent. In addition, during the compounding process, in order to maintain the original aspect ratio, the conductive filler should be strong enough to prevent brittleness due to compounding. However, such a strong conductive filler is very expensive, and is thus not suitable for ordinary low-cost electronic equipment.
The second type of method for preparing conductive component-filled plastic composites involves binding a plastic layer to enclose the conductive filler by immersion or extrusion, and then cutting the conductive long fiber-filled plastic stick to a predetermined length. For example, Japanese Patent No. 60-112854 discloses a process including continuous extruding thermoplastic plastic to enclose a copper fiber so as to form a copper fiber-filled plastic round stick, and then cutting the plastic round stick into pellets of a predetermined size. In order to increase the aspect ratio of the filler, the diameter of the conductive long fiber should be as small as possible. The fibrous filler must be strong enough to prevent breakage. However, such strong fibrous filler, such as stainless steel fiber, copper fiber or metal-coated carbon fiber, is very expensive.
To decrease the total cost involved in the production of conductive component-filled plastic composites, aluminum filler which has the advantages of low price, low density, excellent electromagnetic shielding effectiveness, and ease of color matching has already been used. When aluminum flakes are applied to the first type of method for preparing aluminum-filled plastic composites, the process involves compounding aluminum flakes with plastic. However, since aluminum has low strength, many aluminum flakes break during the compounding process, resulting in a rapid decrease of the aspect ratio. Therefore, the incorporation amount (threshold percolation concentration) should be increased to a very high level (generally, as high as 30 to 40%) to achieve an acceptable electromagnetic shielding effectiveness. The result is that total costs are increased, and more seriously, the electromagnetic shielding plastic products thus obtained have poor mechanical properties. For example, elongation, tensile strength, bending strength and impact strength are all adversely affected.
When aluminum fiber is applied to the second type of method for preparing aluminum-filled plastic composites, the process involves binding a plastic layer to enclose the aluminum fiber by immersion or extrusion. Again, since the diameter of the aluminum fiber is very small and the aluminum has low strength, the aluminum fibers easily break, resulting in a rapid decrease of the aspect ratio.
In order to solve the above-mentioned problems, one of the inventors of the present invention has disclosed in U.S. Pat. No. 5,531,851 a third type of process for making metallized plastic pellets, in which the radially arranged metal is filled. The process involves sandwiching an electrically conductive metal foil in between two plastic films to form a metallized laminated plastic sheet; slicing the plastic sheet into plastic strips; radially arranging the metallized plastic strips into a die of an extruder to be moisturized and bound by the molten plastic into a metallized plastic bar; and finally cutting the plastic bar into metallized plastic pellets of a predetermined size.
Although this third type of process has seen some improvements over the first and second type processes, still, there are some disadvantages to the third type of process, which are outlined below.
1. First, in the third type of process, two rolls of the already-formed plastic film are used, meaning that higher economic costs are incurred with regard to the raw materials.
2. Second, the plastic/metal/plastic laminated sheet obtained from sandwiching a conductive metal foil in between two plastic films is very thick. Thus, it is very difficult to slice the thick laminated sheet into laminated strips of width 1 to 2 mm, and frequently the result is separation between the plastic films and the metal foil.
3. Since the outer layers of the plastic/metal/plastic laminated strips are plastic, when the strips enter the die of the extruder, they immediately melt; thus, since the plastic layers no longer provide any reinforcement to the metal foil, the metal foil must independently take up the tensile stress in the die to a very large extent. This means that the aluminum foil, with low strength, will break easily, and thus th e whole process is interrupted.
4. In addition, due to the size of the inlet of the die of the extruder, the laminated strips that can enter the die at the same time are limited. Only one layer of aluminum foil is contained in each such plastic/metal/plastic strip. If the strip structure can be altered to increase the number of the metal layers in each plastic/metal/plastic strip, the amount of metal foil in the metallized plastic pellet can be effectively increased.
According to the above descriptions, to date, all three types of process for making aluminum-filled plastic composites suffer from some problems. Since aluminum has low strength, in the first and second types of processes, aluminum flakes and aluminum fibers easily break, resulting in a rapid decrease of the aspect ratio. Therefore, the amount of aluminum incorporated should be increased to a very high level to achieve an acceptable electromagnetic shielding effectiveness. The consequence is that total costs are increased, and more seriously, the electromagnetic shielding plastic products thus obtained have poor mechanical properties.
Although the third type of process has achieved some improvements over the first and second type of processes, it still suffers from some problems, such as higher costs with respect to raw materials, separation of the plastic films from the metal foil, easy breakage of the metal foil, and an inadequate amount of metal foil in one plastic pellet.